The present invention relates to a radio communications apparatus for use in a mobile communications system such as a car telephone system and a PHS (personal handyphone system) and, more particularly, to a radio communications apparatus with a combining diversity.
In general, a mobile communication system employs a diversity reception system in order to reduce an influence of fading. For example, a base station of the PHS comprises a plurality of antennas separated from one another, and radio signals received by these antennas are weighted by a weight coefficient set in accordance with their electric field strengths and then combined together. This combined signal is demodulated and then decoded by a CODEC.
FIG. 1 is a block diagram of the arrangement of a radio circuit of a conventional PHS base station.
As illustrated in FIG. 1, radio signals transmitted from a PHS mobile station (not shown), are received by four antennas 1a to 1d and supplied to frequency converters 2a to 2d, respectively. In these frequency converters, the signals are mixed with local oscillation signals generated from local oscillators (LO) 3a to 3d to down-convert them to signals having an intermediate frequency and then to supply the signals to operational amplifiers 4a to 4d for weighting.
The radio signals received by the antennas 1a to 1d are inputted to received signal strength indicators (RSSI) 5a to 5d, respectively to detect their electric field strengths. These electric field strength signals detected by RSSI are supplied to the operational amplifiers 4a to 4d. The operational amplifiers 4a to 4d multiply the intermediate-frequency signals by the electric field strength signals.
In the operational amplifiers 4a to 4d, the intermediate-frequency signals are weighted based on the RSSI detected signals. The weighted four intermediate-frequency signals are combined together into one combined intermediate-frequency signal in an addition circuit 6, and this signal is sent to a demodulation circuit 7 and demodulated therein.
Since, in the foregoing conventional system, the frequency converters 2a to 2d and the RSSIs 5a to 5d differ in processing time from each other, the RSSI signals output from the RSSIs 5a to 5d and the intermediate-frequency signals output from the frequency converters 2a to 2d differ in input timing to the operational amplifier 4a to 4d from each other. Consequently, in the operational amplifiers 4a to 4d, the intermediate-frequency signals are weighted by the RSSI detected signals in different timing. In other words, the intermediate-frequency signals cannot be weighted in accordance with the RSSI signals corresponding to the portions thereof. The conventional system has a drawback in which the intermediate-frequency signals output from the frequency converters 2a to 2d cannot be weighted appropriately thereby to cause a demodulation error and lower communication quality.